This study analyzed the sustainability of different types of in-situ concrete flat slabs used in office buildings in Spain. It found that post-tensioned flat slabs produced the lowest carbon dioxide emissions due to their improved efficiency over traditional flat and waffle slabs. However, post-tensioned slabs remain less common in Spain due to lack of specialized expertise and codes not adequately addressing their design. The study concluded sustainability criteria should be incorporated into codes and construction practices to promote more widespread use of post-tensioned slabs.